All-inclusive resorts have their own tour desks, and if you're staying in a resort, you can reliably use the company that runs the tour desk. It makes sense to use your hotel's tour operator unless you hear negative comments from other guests or if the company doesn't offer the tour you prefer.
The primary tour operator on the Southeast Coast is Tropical Tours (with headquarters at Casa de Campo in La Romana), whose prices are even less than some nonpros and the cruise-ship excursions. Their vans are new or nearly new and well-maintained. If you've ever been on a Dominican excursion when a vehicle breaks down with no mechanic to come to the rescue, you can appreciate this. Also, most of their staff speaks English as well as other languages. They can take you to caves, on quads, on outback safaris, or even out to a ballgame. The company also operates boat tours to the islands.
Tropical Tours (Casa de Campo, La Romana. 809/556-5801. tropicaltoursromana.com.do).